1. Field of the Invention
This invention relates to a broaching tool and an associated method of broaching a plurality of coaxial bearing bores contained within a component for an internal combustion engine.
2. Disclosure Information
The manufacturing processes used for machining internal combustion engine components have included broaching for many years.
U.S. Pat. No. 4,294,568 to Lipowsky et al. discloses a broaching process for producing grooves for retaining blades on the rotor of a gas turbine engine.
U.S. Pat. No. 4,382,731 to Rigdon discloses a rotary broaching device for machining the flat surfaces required on a cylinder block, or cylinder head, of an internal combustion engine.
U.S. Pat. No. 4,693,813 to Takeshita et al. discloses a broaching method for finishing the rotors of a Roots-type blower found many two-stroke cycle Diesel engines.
Finally, U.S. Pat. No. 768,904 to Schmid discloses a broaching machine for finishing cylindrical surfaces, such as an engine crankshaft.
None of the broaching systems cited above is adaptable for machining the bearing bores found in an engine block for crankshaft bearings, or the bores found in cylinder heads or cylinder blocks for camshaft bearings. Although broaching has been used to finish such components as gears and other devices, broaching has not been used for finishing such devices as engine blocks because of difficulty associated with radial drifting of the broach bar as it is moved through axially separated bearing bores, such as those which support a crankshaft or camshaft
At one time, almost all automotive engine blocks, including the crankshaft bore areas, were constructed of cast iron. Commonly, cast iron bearing caps were used for the purpose of retaining the crankshaft and its associated bearings within the engine block. Machining of the crankshaft bearing bores in such case presented no particular problem because the machine tooling was required to work only with cast iron. For several reasons not important to the present invention, automotive manufacturers are now turning in increasing numbers to light metal alloy cylinder blocks made of alumium alloys and other types of alloys. It has been found, however, that light metal alloy bearing caps sometimes have insufficient strength for the demanding requirements of modern engines. As a result, iron alloy main bearing caps are frequently used along with a light metal alloy cylinder block. This type of main bearing bore construction presents a problem inasmuch as the finishing tools traditionally used for bearing bores are not able to maintain truly concentric bearing bores without much difficulty. This fact arises because the light metal alloy cylinder block, being softer than the cast iron bearing cap, will be preferentially removed by traditional cutting tools, and this will cause the bore to drift in the direction of the light alloy cylinder block. Bore drift is undesirable because excessive engine noise will result if the main bearings are not all coaxial.
It is an object of the present invention to provide a tooling system and method for machining main bearing and other coaxial bearing bores contained within a component for an internal combustion engine using a broach with intermittent pilots. A segmented broach bar according to the present invention will provide significant advantages over known systems for finishing coaxial bearing bores.
FIG. 1 illustrates a prior art boring system having a lead element, 12, with a plurality of cutter inserts, 14, located therein. A plurality of support pads, 16, extend axially from the lead end of the boring bar to the powered end of the bar. The bar shown in FIG. 1 has been used for machining bores for engine camshafts. This bar suffers from a potentially serious drawback, insofar that the bar must be directed initially, or "pointed" properly, in order for the resultant finished bores to be finished properly located. Note in this regard that support pads 16 do not serve to guide the boring bar and do not determine the location of the bored holes; the support pads only follow the path created by lead element 12 and cutter inserts 14. Accordingly, errors in tool placement will be perpetuated, if not exaggerated, as the bar moves through successive bearing bores. And, this bar is prone to dimensional errors arising from the problems associated with machining of dissimiliar metals.
FIG. 2 illustrates a boring arrangement, used for machining crankshaft main bearing bores, in which a boring bar, 18, having a plurality of cutter inserts, 20, is simply supported between a boring machine spindle mounting face, 24, and an outboard bushing arrangement, 22. The bar is shown as passing through a series of five main bearing bores, 21. This arrangement suffers from several problems. First, insofar as the boring bar is not piloted at any position intermediate outboard bushing 22 and spindle mounting face 24, the problems associated with dissimilar metal machining will cause the bar to deflect and the resulting bored surfaces will lack the concentricity and roundness generally desirable for use with an engine crankshaft. Moreover, the alignment of outboard bushing 22 is critical if the proper placement of the finished bearing bores is to occur. Also, equipment of this type requires three longitudinal passes to create the final bored surface, and this increases machine time. Finally, the machined surfaces produced by the boring bar are generally too rough for usage without further finishing and must be microsized by a tool such as that shown in FIG. 3. The tool of FIG. 3 includes a bar, 26, having diamond inserts, 28, which are used to microsize, or finish, a previously machined bore. The microsizing bar generally removes approximately 0.0002 to 0.0005 inches of material on diameter. If the stock removal is increased above this rate, the bore's centerline will begin to distort due to aluminum being preferentially removed. In sum, prior art boring systems require multiple passes with different tooling and do not produce adequate results in terms of precision and repeatability, particularly when dissimilar metal machining is involved.
It is an object of the present invention to Provide a machining system for crankshaft bores, camshaft bores and other axially separated bores within internal combustion engine components which will produce coaxial bearing bores having true circular configurations.
It is another object of the present invention to provide a machining system for multiple bearing bores which will produce the finished bore with one pass of a machining tool, as opposed to multiple passes required by prior art devices.